Brake systems of motor vehicles are safety-relevant assemblies and have to be checked regularly, both in the manufacturing process at the automobile manufacturer's plant and during later inspections of the vehicle. The brake systems are tested during manufacturing, particularly after they have been filled with operating fluids. It is the purpose of the test to detect any existing leaks or entrapped air in the system. At the same time, the tests are to ensure a defined position of the pedal system and easy operation of the moving parts in the brake system. Various technical solutions for such tests are known from prior art.
DE 100 09 168 A1, for example, describes a device with magnetically sensitive components for detecting a pedal operating force applied by the driver. The device includes a carrier plate with an associated tongue element which is mounted to the brake pedal using bolts. When the driver's foot applies a force to the brake pedal, the relative position between the tongue element and the carrier plate changes. Therefore the magnetic field that is present in this area also changes depending on the force applied to the pedal. This allows a statement about the pedal operating force, but without capturing the pedal travel.
DE 44 24 094 C1 however discloses a device, the application of which also considers pedal travel. A sensor that is connected to an evaluation unit detects pedal movements.
DE 2 250 728 C describes a pedal force meter that includes a housing, a step plate to which a pedal operating force can be applied, and a device for outputting a signal depending on the pedal operating force. The housing includes a base plate designed as an adapter with which the entire device can be detachably attached to the brake pedal. The base plate forms a contour together with a circumferential frame that is closed at the top by the step plate. Measuring elements are arranged in the interior to determine the application of force to the pedal plate.
A measuring device known from design registration DE 403 06 723.5 includes force and travel sensors and performs activations of the brake pedal. In a first preset stroke, the brake system is first prepared for the test. In a subsequent measuring stroke, a travel-force characteristic is recorded from which the device determines a pedal travel at a test force. This characteristic allows the detection of any disruptions, e.g, air entrapped in the hydraulic system or a malfunction of an assembly. Then a leak test is performed to evaluate if the brake system is leaking or not.
Although various technical solutions for testing brake systems of motor vehicles are known, there continues to be a need for development. A major aspect is that the evaluation of the brake systems using the common brake pedal testing equipment has been limited to evaluating pedal travels and pedal forces with respect to any residual air or leakage when operating the brake pedal. But such evaluation has yet to take into account that the respective brake pedal travel is the sum total of a multitude of design factors that influence the brake system components. The residual air that is primarily detected is just one of the influencing factors.
Therefore the formerly common method of evaluating the residual air inevitably resulted in a high proportion of false evaluations, which can either be a false “not ok” (hereinafter called “NOK”) or a false “ok” (hereinafter called “OK”). Accordingly, vehicles are either unnecessarily taken off the assembly line for reworking due to a false NOK rating, or they are rated OK despite an inadmissibly high proportion of residual air and not reworked. Studies by the applicant have shown that this proportion can be up to 30%, wherein the percentage of actually accurate evaluations as NOK is typically smaller than 1%.